1. Field of the Invention
The present invention generally relates to an improved comb filter system and method, and in particular an improved system and method for separation of composite signals.
2. Background
Due to limitations on available bandwidth and the increased demand to transmit additional information on existing bandwidth, it is often necessary to multiplex or combine two or more information signals into a single composite signal.
A color television signal is an example of a composite signal. A color television signal comprises a luminance (brightness) component and a chrominance (color) component. These components are often represented as Y and C components wherein Y represents the luminance component and C represents the chrominance component.
Originally, broadcast television in the United States began with black and white broadcast and therefore lacked the chrominance component, C, of modern television's composite signal. Television standards and technology required that the black and white television signal, that is, the luminance component (Y), reside within 6 megahertz (MHz) of bandwidth space.
Eventually, however, technology advanced to provide color television. To allow black and white televisions to receive the new color signal broadcast, the color signal standard located the color information within the same 6 MHz of bandwidth space allotted to each channel of the black and white signal. Under this standard, the color information overlaps with the luminance information.
FIG. 1 illustrates a composite television signal on a coordinate system in which the horizontal axis 100 represents frequency and the vertical axis 102 represents amplitude. Signal line 104 represents the luminance information (Y) while line 106 represents the chrominance information (represented as I and Q) of the composite signal. As shown, the frequencies of these signals 104, 106 overlap. In an NTSC (National Television Standards Committee) system, the luminance information occupies the range DC to 5.5 MHz of bandwidth while the chrominance signal is bandlimited to the range approximately 0.6 to 1.3 MHz and is modulated onto a carrier at 3.58 MHz. The audio portion of the signal is at 4.5 MHz. While these two data signals conveniently fit within the 6 MHz of bandwidth space they are allotted, obvious decoding challenges are presented in order to separate the luminance information from the chrominance information.
The first decoding scheme adopted to separate the overlapping luminance (Y) and chrominance (C) signals comprises simple notch filtering in combination with band pass filtering. FIG. 2 illustrates a block diagram of a basic notch filter 152 and band pass filter 154. An incoming composite signal on line 150 is presented to both of the notch filter 152 and the band pass filter 154.
FIG. 3 illustrates the frequency response of a notch filter 152 and a band pass filter 154. The output of the notch filter generally comprises the luminance portion 174 of the composite signal while the output of the band pass filter generally comprises the chrominance portion 176 of the composite signal.
In particular, for NTSC video, the notch filter removes a portion of the composite signal centered at 3.58 MHz, but allows the remainder 174 to pass. While the notch filter 152 allows the majority of the luminance information 174 to pass, it undesirably removes components of the luminance signal having frequencies within the range of the notch filtered frequencies 177. The notch filtered frequencies that are removed range from 2.5 to 4.5 MHz. Stated another way, the notch filter allows the frequency band below 2.5 MHz and the frequency band above 4.5 MHz to pass.
The band pass filter 154 configured to operate in accord with the NTSC standard video allows a 2 MHz portion of the composite signal centered at 3.58 MHz to pass while removing portions outside of this band. This portion of the composite signal contains all the chrominance information. Undesirably, however, the output of the band pass filter also contains luminance components having frequencies within the band pass filter's frequency band.
Notch and band pass filtering suffers from numerous drawbacks as can easily be understood with reference of FIG. 3. In particular, the band pass filtered chrominance portion of the composite signal also contains luminance information, i.e., band pass filtering does not remove all luminance information from the chrominance signal. The unwanted luminance information in the chrominance signal introduces artifacts into the video image. This is most noticeable in pictures that contain closely spaced black and white lines, such as when the video display is of person is wearing a herringbone jacket.
Likewise, notch filtering the composite signal to remove the chrominance information from the composite signal to obtain the luminance information removes valuable portions of the luminance signal. A loss of luminance information is especially critical due to the human eye's sensitivity to brightness and contrast variations in a projected image.
Therefore, a need exists for a method and apparatus for video separation that is more robust than prior systems, requires less memory, and more completely separates the components of the composite signal.